1. Field of the Invention
The present invention generally relates to methods and systems for determining one or more properties of a specimen. Certain embodiments relate to methods and systems for determining a minority carrier diffusion length and contamination levels of a specimen. Other embodiments relate to methods and systems for detecting defects on a specimen.
2. Description of the Related Art
Methods and systems are currently available for the measurement of minority carrier lifetimes in semiconductor wafers. The minority carrier lifetimes of semiconductor wafers is of interest to semiconductor manufacturers since the minority carrier lifetimes indicate the degree of contamination in the wafers. For example, the minority carrier lifetimes are inversely related to the degree of contamination in the wafers. In particular, as described in U.S. Pat. No. 6,512,384 to Lagowski et al., which is incorporated by reference as if fully set forth herein, the minority carrier diffusion length, L, is the effective distance that excess minority carriers diffuse into a semiconductor during their lifetime. The value of the minority carrier diffusion length is used as an indicator of the purity of semiconductor materials. L gives a measure of the contaminant concentration in the semiconductor because heavy metals function as recombination centers which reduce the minority carrier lifetime. As a result, higher concentrations of contaminants decrease the minority carrier diffusion lengths.
Typically, the diffusion length in silicon wafers is measured at various stages of fabrication of microelectronic chips or “integrated circuits” to measure the concentration of potentially harmful impurities, which may have been inadvertently introduced into the wafer. Frequent monitoring of the minority carrier diffusion lengths helps to identify when a given process or a given tool starts to contaminate wafers above a permissible level. Preventive maintenance of processing equipment or replacement of chemicals done at this stage helps to reduce the possibility of large scale manufacturing losses.
In the past, methods for measuring minority carrier lifetimes of semiconductor wafers have used illumination of the wafers with several wavelengths and detection of the resulting surface photo voltage (SPV) signal, which can then be used to determine minority carrier diffusion lengths. The term “surface photo voltage” is generally defined as a reduction of the surface space charge width during illumination and its recovery in the dark. One example of such a method is illustrated in U.S. Pat. No. 4,333,051 to Goodman, which is incorporated by reference as if fully set forth herein. Goodman uses a broadband light source coupled to a monochromator to illuminate a wafer. The wafer is illuminated with light of different wavelengths sequentially. At each wavelength (corresponding to a different absorption coefficient, α), the photon flux (Φ) is adjusted to achieve the same SPV signal. Modulation of the light source is achieved using a chopper wheel. The diffusion length is determined from a plot of Φ vs. 1/α. Another method was initially developed by Quilliet and one example of a similar method is illustrated in U.S. Pat. No. 5,025,145 to Lagowski, which is incorporated by reference as if fully set forth herein. Quilliet used a broadband light source and monochromator to illuminate a wafer at different wavelengths. In contrast, Lagowski uses a broadband light source and bandpass filters to illuminate a wafer at different wavelengths. Each method maintains a constant photon flux and measures the SPV at different selected wavelengths sequentially. Modulation is achieved using a chopper wheel. In another example illustrated in U.S. Pat. No. 5,663,657 to Lagowski et al., which is incorporated by reference as if fully set forth herein, Lagowski et al. introduced modifications to the voltage probe design, corrections for wafer reflectivity, and corrections for long diffusion lengths (greater than wafer thickness). In a newer version of the constant photon method illustrated in U.S. Pat. No. 6,512,384 to Lagowski et al., Lagowski et al. uses two wavelengths modulated at two different frequencies to perform the constant photon flux determination of diffusion lengths by illuminating the wafer with both wavelengths simultaneously. The first wavelength of light is provided by a broadband light source coupled to a bandpass filter. The second wavelength of light is provided by an infrared light emitting diode (IR LED) coupled to a bandpass filter.
The above described methods and systems, however, have several disadvantages. For example, the systems described in U.S. Pat. No. 4,333,051 to Goodman have the disadvantage that the photon flux has to be adjusted at each wavelength to achieve the same SPV signal. Even using a servo to achieve this adjustment, there is a throughput disadvantage due to the time required to measure the SPV signal and adjust the flux accordingly. The systems described in U.S. Pat. No. 5,025,145 to Lagowski and U.S. Pat. No. 5,663,657 to Lagowski et al. have the disadvantage that the wavelengths are sent to the wafer sequentially. This configuration results in a throughput loss, as the same measurement site has to be sequentially illuminated with each wavelength. There is also a possibility that characteristics of the wafer will change on the time scale it takes to change among the wavelengths. For example, as described by Lagowski et al. in U.S. Pat. No. 6,512,384, the delay, typically 6 seconds, between sequentially measuring the first and second SPV results in changes in the surface condition of the wafer, e.g., static charge created by wafer motion, surface relaxation after previous chemical treatments, and adsorption or desorption caused by ambient changes. These changes alter the SPV signal value and thereby create errors in diffusion length measurements. The systems described in U.S. Pat. No. 6,512,384 address the throughput concern by simultaneously illuminating a wafer with two different wavelengths of light. However, these systems have the disadvantage that the wavelengths are modulated at two different frequencies. This modulation scheme requires the demodulation of the resulting ac-SPV signal at two reference frequencies. Also, SPV amplitudes tend to decrease with increasing modulation frequency. This frequency dependence requires the use of one of two correction procedures shown in U.S. Pat. No. 6,512,384 to normalize the measured SPV signals to a single modulation frequency.
Accordingly, it may be desirable to develop systems and methods for determining a property of a specimen such as the minority carrier lifetime or minority carrier diffusion length which have relatively high throughput, relatively good accuracy, and relative simplicity of design.
Inspection of specimens such as wafers is typically performed at various stages of a semiconductor fabrication process. Inspection is performed to detect defects such as electrical defects and/or any number of other types of defects, which may have been inadvertently formed on the wafer, some of which may adversely affect the performance of integrated circuits that are eventually formed on the wafer. Frequent inspection of wafers helps to identify when a given process or a given tool is causing defects on wafers. Preventive maintenance of processing equipment or replacement of chemicals done at this stage may help to reduce the possibility of large scale manufacturing losses due to the defects.
Electrical defects are of particular interest to those involved in integrated circuit manufacturing since integrated circuits are inherently electrical by nature. Many different methods and systems have been developed to inspect wafers for electrical defects. Examples of attractive methods and systems for electrical defect inspection are illustrated in U.S. Pat. No. 6,445,199 to Satya et al. and U.S. Pat. No. 6,642,726 to Weiner et al., both of which are incorporated by reference as if fully set forth herein. One method described by Satya et al. includes illuminating a sample with a charged particle beam thereby causing voltage contrast within structures present on the sample. The sample may be illuminated using a scanning electron microscope system. In the voltage contrast mode, the scanning electron microscope can be used to distinguish charge floating conductor shapes from charge-drained grounded shapes in terms of visual or intensity contrasts in the voltage contrast data. Position data concerning the location of electrical defects may also be determined from the voltage contrast information.
In one method described by Weiner et al., a test structure is designed to include a plurality of features that will charge to specific voltage potentials when scanned with an electron beam during a voltage contrast inspection. Images of the scanned features are generated, and the relative brightness level of each feature depends on the corresponding potential of each feature during the inspection. That is, some features are expected to appear dark, and other features are expected to appear bright. If there is no defect present in the scanned feature, the corresponding image will have the expected number of bright and dark features. However, if there is a defect present, the number of dark and bright features within the generated image will not match expected results.
Although the methods and systems described by Satya et al. and Weiner et al. have proven to be extremely useful for detection of electrical defects, particularly in comparison to other electrical defect inspection methods and systems, certain aspects of these voltage contrast-based inspection methods and systems allow room for improvement. For example, since these methods and systems use a charged particle beam for inspection, inspection must be carried out in a vacuum environment. Performing inspection in a vacuum environment is more expensive and slower than performing inspection in an ambient environment. In addition, the electrical defect inspection methods and systems are generally designed for inspection using only one type of charged particle (e.g., electrons). Therefore, the data that can be generated by such inspection methods and systems may be somewhat limited, particularly for use in identification of defect types (i.e., classification).
Accordingly, it may be desirable to develop methods and systems for electrical defect inspection and classification that can be used in an ambient environment thereby reducing the cost and increasing the throughput of electrical defect inspection and that can generate more useful data than those methods and systems that are currently available for electrical defect inspection.